Thermal insulation for saving energy has attained great prominence in the context of desire for sustainable development and the increasing cost of energy. Thermal insulation is gaining ever greater importance in the light of increasing energy prices, increasingly scarce resources, the desire for reducing CO2 emissions, the necessity of a sustainable reduction in energy demand and also the increasingly demanding requirements which protection against heat and cold will have to meet in the future. These increasingly demanding requirements for optimizing thermal insulation apply equally in buildings, e.g. new buildings or existing buildings, and to thermal insulation in the mobile, logistics and stationary sectors.
Building materials such as steel, concrete, masonry and glass and also natural rock are relatively good thermal conductors so that the exterior walls of buildings made thereof very quickly give off heat from the inside to the outside in cold weather. Development is therefore aimed, firstly, at improving the insulation properties by increasing the porosity of these building materials as in the case of, for example, concrete and masonry, and secondly at cladding the outer walls with thermal insulation materials. The thermal insulation materials which are mostly used at present are materials having a low thermal conductivity. Materials used include both organic insulation materials and inorganic insulation materials, e.g. foamed plastics such as polystyrene, and polyurethane; wood fiber materials such as wood wool and cork; vegetable or animal fibers such as hemp, flax, wool; mineral and glass wool, foamed glass in plate form; calcium silicate boards and gypsum plasterboards. Those thermal insulation materials are mostly used in the form of foamed or pressed boards and moldings, alone or in combination with others. Another effective way to provide thermal insulation is the use of vacuum insulated panels (VIPs) which are based on the principle of vacuum insulation. Those VIPs comprise a porous core material to support the vacuum and surrounded by a highly gas-tight cover material. Materials that may be employed for the core include open-cell polymer foams, microfibre materials, fumed silica and perlite.
The insulating capability of each of the above-mentioned materials and vacuum/material combinations, respectively, can be further improved by adding an athermanous material capable of interacting with infrared radiation and thus reducing infrared transmission. For example, athermanous materials may be used as fillers in thermoinsulating polymeric foams and in vacuum insulated panels. Expandable thermoplastic polymers and among these, in particular, expandable polystyrene (EPS), are conventional insulation materials which have been known and used for a long time for preparing expanded articles which can be adopted in various applicative areas, among which, one of the most important is thermal insulation. The flat sheets of expanded polystyrene are normally used with a density of about 30 g/l as the thermal conductivity of the polymer has a minimum at these values. It is not advantageous to fall below this limit, even if this is technically possible, as it causes a drastic increase in the thermal conductivity of the sheet which must be compensated by an increase in its thickness. In order to avoid this drawback, the polymer can be filled with athermanous materials such as graphite (e.g. in Neopor® available from BASF), carbon black or aluminum. A good performance of the athermanous filler and thus of the overall thermal insulation allows a significant reduction in the density of the expanded article or thickness of the same without reducing the overall thermal resistance value.
EP 620,246 A describes a process for preparing granules of expandable polystyrene containing an athermanous material, for example carbon black, distributed on the surface or, alternatively, incorporated inside the particle itself.
The use of carbon black has long been known as a filler or pigment, or else as a nucleating agent (see, for example, Chem. Abstr., 1987, “Carbon Black Containing Polystyrene Beads”). Among the various types of carbon black, the most important are carbon black from oil combustion (“petroleum black”)/carbon black from gas combustion, carbon black from acetylene, lamp black, channel black, thermal black and electrically conductive carbon black. WO 1997/45477 describes compositions based on expandable polystyrene comprising a styrene polymer and from 0.05 to 25% of carbon black of the lamp black type.
Depending on the manufacturing process, these carbon blacks have diameters which range from 10 nm to 1,000 nm approximately, and have very different specific surfaces (from 10 to 2,000 m2/g). These differences lead to different blockage capacities of the infrared rays. WO 2006/61571 describes compositions based on expandable polystyrene comprising a styrene polymer and from 0.05 to less than 1% of carbon black, with a surface area ranging from 550 to 1,600 m2/g.
It is known that graphite can also be effectively used as a black body (as described, for example, in JP 63-183941, WO 04/022636, WO 96/34039). Its use as attenuating agent of infrared radiation in polymeric foams is, however, more recent. Patent application JP 63-183941 is among the first to propose the use of some additives, active in blocking infrared rays in wavelengths ranging from 6 to 14 μm, and therefore obtaining thermally insulating thermoplastic resins capable of permanently sustaining a low thermal conductivity. Among all additives, graphite is preferred.
DE 9305431 U describes a method for producing expanded molded products having a density of less than 20 kg/m3 and a reduced thermal conductivity. This result is reached by incorporating an athermanous material, such as graphite and also carbon black, in the rigid polystyrene foam. International patent application WO 98/51735 describes expandable polystyrene particulates containing 0.05 to 25% by weight of synthetic or natural graphite particles, homogeneously distributed in the polystyrene matrix. The graphite preferably has an average diameter ranging from 1 to 50 μm, an apparent density ranging from 100 to 500 g/l and a surface area ranging from 5 to 20 m2/g.
WO 2011/042800 is directed to an expandable thermoplastic nanocomposite polymeric composition, preferably a polystyrene composition, including an athermanous filler comprising nano-scaled graphene sheets having a thickness not greater than 150 nm, an average dimension (length, width, or diameter) not greater than 10 μm and a surface area >50 m2/g.